1. Field of the Invention
The present invention relates to measuring and monitoring steam quality. In particular, the present invention relates to a system and method to detect water droplets in steam. Even more particularly, the present invention relates to optical measurement of water droplets in steam for adjustment of steam quality of the steam being produced.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Steam quality is defined as the proportion of saturated steam vapor in a liquid/vapor mixture. A steam quality of zero indicates 100% liquid, while a steam quality of one hundred percent indicates 100% steam vapor. Low steam quality affects steam system operations in several ways. In particular, low steam quality in heat transfer processes can reduce heat transfer efficiency by more than 65%. The liquid that is entrained in the steam has a significantly lower amount of energy than the steam vapor latent energy, and therefore low steam quality results in less usable energy being delivered to the steam process equipment. Further, the additional liquid that is present in the steam collects on wetted surfaces of the heat exchanger equipment, causing a buildup of liquid that reduces the ability of the heat exchanger to transfer the latent energy of the steam to a product. In addition, liquid passing through steam control valves can erode the internals of the valves, thereby contributing to premature valve failure. Similarly, the liquid that is introduced with the steam into a saturated turbine operation can reduce the life expectancy of the internal turbine components. Finally, since steam systems are not typically designed to accommodate the additional liquid that is present in low quality steam, the presence of the additional liquid increases the chance for a pressure surge, when a valve closes at the end of a pipeline, known as a water-hammer, to occur. Water-hammer is a safety issue and may cause premature failure in the steam system.
In other applications, such as thermally enhanced heavy oil recovery, steam is injected into the ground to lower the viscosity of the oil. Generated steam is required for the enhanced oil recovery process. The generated steam is monitored for steam quality because steam quality has an effect on the oil recovery process and efficiency. It is typical for a lower quality steam to be generated for enhanced oil recovery processes. Typically, treated re-used produced water is fed into the boiler. Due to the poor quality of the boiler-feed water (BFW), the maximum allowable steam quality to avoid boiler tube failure in Once through Steam Generators (OTSG) is only about 80%. On the other hand, drum boilers can operate at 100% steam quality but are only suitable for operation at much lower pressures than OTSG boilers, because at higher pressures they can experience tube deposition. The high-pressure conditions that are encountered during thermally enhanced heavy oil recovery operations often preclude the use of drum boilers. The generated steam for the enhanced oil recovery process must remain within a desirable range of steam quality. Steam quality can be monitored and controlled.
In current boiler operations the temperature, pressure and single-phase flow are measured and can be accurately derived. Unfortunately these readings do not give accurate quality measurements for the two-phase (vapor and liquid) steam coming out of the boiler, since 0% and 100% quality steam can have the same temperature and pressure. A throttling calorimeter is useful for certain steam quality measurements, but this method is not suitable for wet steam or for the high pressures that are used in the heavy oil industry.
Two methods are known for measuring steam quality online. The first uses a flow meter and Bernoulli's principle to calculate volumetric flow rates. This method is not accurate and has issues of working only for a short time due to clogging and deposition. The second online approach is based on mass flow calculations, and involves measuring the flow rate of the Boiler Feed Water (BFW) and then dividing the flow rate by the blow down flow rate. This latter method is also not accurate, and producers are having trouble getting the numbers to add up, likely because just measuring the blow down flow rate in the steam separator does not ensure ideal separation of the steam and liquid. Furthermore, this method does not give an indication of steam quality in each pass of the boiler.
Specific conductance measurement is a commonly used offline technique, which is based on the principle that the conductance of the water is proportional to the concentration of ions in the sample. When liquid boiler feed water is carried over in steam, the dissolved solids content of the boiler water contaminates the steam, and as a result the steam sample conductivity increases. A disadvantage of using specific conductance measurements is that some gases that are common to steam (such as carbon dioxide and ammonia) ionize in water solution. Even at extremely low concentrations, the ionized gases interfere with measurement of dissolved solids by increasing the conductivity.
Other known approaches for determining steam quality include optical monitoring methods, as taught in the following patents: U.S. Pat. No. 8,433,526, Method and system for steam quality monitoring; U.S. Pat. No. 7,381,954, Apparatus and method for measuring steam quality; U.S. Pat. No. 7,345,280, Measurement of steam quality using multiple broadband lasers; U.S. Pat. No. 7,034,302, Optical steam quality measurement system and method, and U.S. Pat. No. 4,137,462, Probe for measuring steam quality.
The prior art uses an emitter to send either a single wavelength or plural wavelengths of light through a steam conduit to a receiver, the emitter and the receiver being directly lined up one with the other. In each of the methods the intensity of light that is incident on the receiver is related back to steam quality, and various approaches of sending wavelengths that are more and less sensitive to liquid and vapor water are used to extract information via the Beer-Lambert law. For instance, U.S. Pat. No. 8,433,526 discloses a method for determining droplet size using a lined up emitter/receiver configuration, in which the intensity of light that is incident on the receiver when a dry steam is measured is compared to the intensity of light that is incident on the receiver when a wet steam is measured. The droplet size is determined based on the intensity drop measured at the receiver, and is related to total scattering based on Mie Scattering theory.
FIGS. 1A and 1B illustrate three types of scattering from a liquid droplet in a steam conduit. In particular, FIG. 1A illustrates scattering due to diffraction 3 when a beam of light 1 interacts with a water droplet 2, and FIG. 1B illustrates scattering due to reflection 6A and scattering due to refraction 7 when a beam of light 4A interacts with a water droplet 5. It is important to note that scattering due to diffraction and reflection contain no absorptive information relating to the liquid droplet, and that only scattering due to refraction contains absorptive information. A closed solution for the scattering of a plane wave from a spherical, homogeneous, isotropic particle (in this case a spherical water droplet) was first presented by Lorenz in 1890 and by Mie in 1908, which is known as the Lorenz-Mie Theory (LMT). The LMT solution is documented in the literature by Born M., Wolf E “Principles of Optics” Cambridge University Press London, Born M. “Optik” Springe, Verlag Berlin 1981, Kerker D M. “The Scattering of Light” Academic Press, New York, London 1969 and Boheren C F, Huffman D R “Absorption and Scattering of Light by Small Particles” Wiley 2007. An extension using the Debye series was also documented by Debye in 1908 and Hovenac E A, Lock J A “Assessing the contribution of surface waves and complex rays to far field Mie Scattering by use of Debye Series” Journal of Optical Society America, Volume 9, pp. 781-795, 1992, which allows the computed scattered field to be interpreted in terms of scattering orders.
FIG. 2 presents a summary of the intensity distribution of the first 10 scattering orders of water droplets in air, as calculated using LMT and Debye Series. Diffraction dominates the signal that is detected at the receiver with a scattering angle of zero, i.e. when the receiver is lined up directly with the emitter. A contribution due to refraction is present at zero degrees scattering angle, but it is two orders of magnitude lower than the diffraction contribution. For this reason, the perceived loss of optical power at the receiver is dominated by diffraction at zero degrees scattering angle.
The diffraction of a homogenous wave from a spherical droplet can be approximated using Fraunhofer diffraction from a circular disc. The Intensity I(θ)Diff due to diffraction at a point on a lined up emitter/receiver is given by equation (1):
                                          I            ⁡                          (              θ              )                                Diff                ∝                              [                                                            J                  1                                (                                  π                  ⁢                                                                          ⁢                  D                  ⁢                                                                          ⁢                  s                  ⁢                                                                          ⁢                                      in                    ⁡                                          (                                              θ                        λ                                            )                                                                                                  π                ⁢                                                                  ⁢                D                ⁢                                                                  ⁢                s                ⁢                                                                  ⁢                                  in                  ⁡                                      (                                          θ                      λ                                        )                                                                        ]                    2                                    (        1        )            Where J1 is the Bessel Function of the first kind of order one, D is the diameter of the droplet and θ is the convergence angle. So when multiple wavelengths, some of which are more sensitive to absorption in liquid compared to vapor and vice versa, are sent through a steam conduit in a lined-up emitter/receiver configuration, all wavelengths will experience a large intensity drop due to diffraction and the wavelengths which are more sensitive to absorption in liquid will experience a much smaller change compared to the wavelengths which are not sensitive to absorption in liquid. With decreasing steam quality the intensity drop becomes increasingly large, due to the larger size of the water droplets and/or due to the larger number of water droplets present in the steam, which places a lower limit on the steam quality that can be measured using a lined-up emitter/receiver configuration. This can be seen in the published results of J. K. Partin, J. R. Davidson (Idaho National Laboratory August 2006), where the optical signal was near zero for 99.4% steam quality by mass.
It would be beneficial to provide a system and method that overcomes at least some of the above-mentioned limitations.
It is an object of the present invention to provide an embodiment of a measurement system to collect absorptivity information relating to liquid water droplets entrained in steam in steam system operations.
It is another objection of the present invention to provide an embodiment of an online optical monitoring and measurement system to derive water vapor content, as well as the velocity, directional velocity, shape and size of liquid water droplets entrained in steam in steam system operations.
These and other objectives and advantages of the present invention will become apparent from a reading of the attached specification.